Cause Erysiphe necator (formerly Uncinula necator), a fungal disease common to all areas of the Pacific Northwest. The disease tends to be more severe on the west side of the Cascade Range but is a chronic problem in arid districts where over-the-canopy irrigation is used for early-season frost protection or watering. There is low tolerance to this disease on the fruit as it can impart negative tastes later in the wine. An infection of 3% to 5% of the berries results in an oily, viscous mouth feel in the wine. Vitis vinifera (European) cultivars are susceptible to powdery mildew while V. labrusca (American) cultivars are more resistant. Hybrids between the two species have various levels of resistance. Other hosts include Boston ivy, Virginia creeper, and Ampelopsis (porcelain berry). The fungus may overwinter as a group of thin threads called hyphae, inside the vine's dormant buds and/or as small black bodies (chasmothecia) on the exfoliating bark of the vine.
Buds on new shoots can be infected 4 to 6 weeks after shoots start growing but not after bud scales become suberized. These new infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season. Shortly after budbreak, the fungus becomes active and covers the emergent shoot with a large white mass of threads or mycelium ("flag shoots"). Flag shoots have rarely been observed in commercial western Oregon or eastern Washington vineyards.
Chasmothecia on the exfoliating bark release sexual spores during rainy weather above 50°F from budbreak through bloom. This weather also favors infection that results in individual powdery spots, called colonies, on the surface of leaves growing close to the bark.
Many asexual spores (conidia) are produced on the surface of powdery mildew colonies. Under optimal conditions of mild temperatures and high humidity, a single spore can germinate, infect the plant, and produce a new colony and a new crop of spores in 3 days. Temperatures over 85°F and/or sunlight (ultraviolet) inhibit conidia germination. Nights where temperatures fall below 40°F can damage colonies, increase the time new ones form, and reduce their size. Free water from rain and/or irrigation can wash conidia off a colony, burst conidia, or result in poor or abnormal germination of conidia.
Grape berries are highly susceptible from the time calyptras (hoods) fall off to shortly after bloom when berries are about pea size (BBCH 73 to 75). Susceptibility of the fruit drops rapidly after that time. Grapes do not get new infections on fruit after 8% Brix but can still have sporulation up to 15% Brix. Leaves and canes, however, can be infected up to and past harvest.
Symptoms Powdery mildew can attack all aboveground plant parts. In early stages, whitish or grayish patches are on leaves and, if severe, ultimately cover both surfaces. Colonies are more easily detected in full sunlight with the sun over your shoulder. Later in the season, the colony darkens and is peppered with minute black dots (chasmothecia). On fruit, the fungus at first may look grayish or whitish but later has a brownish, russeted appearance. Infected fruit cracks and drops from the cluster. Even blossoms sometimes can be infected, causing them to dry up or fail to set fruit. When green shoots and canes are infected, the affected tissues appear dark brown to black in feathery patches. Patches later appear reddish brown on the surface of dormant canes.
Flag shoots are difficult to detect. Some young shoots may be covered with a large white mass of threads or mycelium. Others may have only a hint of thin threads on the shoot. Shoots generally are delayed in bud break and appear stunted and somewhat yellowed compared to healthy shoots. Can be confused with normal trichome density of young shoots of some cultivars.
Cultural control Activities and tactics that increase sunlight exposure to leaves and fruit reduce powdery mildew severity.
- Prevent excess vigor through proper selection of rootstocks, training systems, irrigation, and fertility for the vineyard site.
- Practice timely sucker control.
- Keep canes cut back close to the top wire of vertical trellises for better spray coverage.
- Removing leaves to manage bunch rot also helps fungicide cover the clusters, which helps control powdery mildew.
- Manage sources of powdery mildew outside of the vineyard such as susceptible plants found in decorative gardens near tasting rooms or gazebos or volunteer vines along fence rows or riparian areas.
Chemical control Begin applications at 6 inches shoot growth and continue at regular intervals through the growing season. Strongest materials and shortest intervals should be used from prebloom (BBCH 57) through bloom and continued for 2 weeks after bloom. Adjusting application timing to coincide with early bloom (BBCH 61 to 65) has resulted in more consistent control of powdery mildew in western Oregon. A postharvest application may help control late-season infections in some years. Thorough coverage of all actively growing tissue is essential for good control. Do not extend application intervals in the Willamette Valley past 2 weeks.
Research in California has shown that sulfur sprays at budbreak initiate a release of ascospores and have some utility in overall disease management. Although this may occur in the more arid areas of the Pacific Northwest, it may not have the same utility in wetter areas west of the Cascade Range.
Resistance to many different fungicide groups has been documented for grape powdery mildew worldwide. Resistance to Group 3 and 11 fungicides has been documented in Oregon and Washington. To prevent developing resistant fungi, alternate or tank-mix materials from different groups with different modes of action. Also, limit applications from any specific group to two (2) or fewer sprays. New York recommends to tank-mix Group 3 or Group 11 fungicides with sulfur or other products with a different mode of action. Several forecasting programs are available to help time applications (see below).
- Abound at 10 to 15.5 fl oz/A. Do not apply within 14 days of harvest or with silicone-based surfactants. Sprayers used to apply Abound should not be used on apples. Group 11 fungicide. 4-hr reentry.
- Aprovia at 8.6 to 10.5 fl oz/A plus an adjuvant. Do not use within 21 days of harvest. Group 7 fungicide. 12-hr reentry.
- Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Only use early season since thorough coverage is essential and timed when disease pressure is low. O
- Kaligreen (82% potassium bicarbonate) at 2.5 to 5 lb/A. 4-hr reentry.
- MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A. Oregon and Washington only. 1-hr reentry.
- Monterey Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water. H
- Cinnerate at 13 to 35 fl oz/100 gal water. 4-hr reentry.
- Copper formulations are registered but provide only moderate control alone. They are not generally recommended. O
- Eagle 20 EW at 6 to 10 fl oz/A for home vineyard or landscape use. Do not apply within 14 days of harvest. Group 3 fungicide. 24-hr reentry.
- Endura at 4.5 oz/A. Do not use within 14 days of harvest. Do not use for powdery mildew if you plan to use it for bunch rot. Group 7 fungicide. 12-hr reentry.
- Flint 50 WG at 1.5 to 2 oz/A. Do not use on 'Concord' grapes. Do not use within 14 days of harvest. Group 11 fungicide. 12-hr reentry.
- Fracture (BLAD) at 20.5 to 24.4 fl oz/A. Reapply if rain occurs within 12 hours of original application. Do not use within one day of harvest. Group BM01 fungicide. 4-hr reentry.
- Gatten at 6.4 fl oz/A. Do not use within 14 days of harvest. Group U13 fungicide. 12-hr reentry.
- Horticultural Mineral Oils. Generally effective from 1% to 2% volume to volume. Necrotic foliage may result if applied within 10 days of any sulfur application. Do not tank-mix with copper-based products when fruit is present. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet because good coverage is essential. Do not use before bloom as it will have a negative impact on predatory mites. 4-hr reentry. O
- JMS Stylet Oil at 1 to 2 gal/100 gal water. Brix reductions have been observed in several locations when product is used all season.
- SuffOil-X at 1 to 2 gal/100 gal water.
- Trilogy at 1% of spray volume but do not exceed 2.5 gal/A. Do not use after bunch closure. Poor to fair control as a stand-alone product.
- Kenja 400 SC at 20 to 22 fl oz/A. Do not use within 14 days of harvest. Group 7 fungicide. 12-hr reentry.
- Mettle 125 ME at 3 to 5 fl oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- M-Pede at 1 to 2 gal/100 gal water. Good coverage is essential. Do not use within 3 days of applying sulfur or past verasion. Do not mix with hard water. 12-hr reentry. O
- Oso SC at 3.75 to 13 fl oz/A. Do not use within 7 days of harvest. Group 19 fungicide. 4-hr reentry.
- Ph-D WDG at 6.2 oz/A plus an adjuvant. May be applied on the day of harvest. Group 19 fungicide. 4-hr reentry.
- Prev-Am Ultra at 50 fl oz/100 gal water. Do not use within 14 days of a sulfur application, above 90°F, or when plants are under heat or moisture stress. 12-hr reentry.
- Procure 480 SC at 4 to 8 fl oz/A. Do not use within 7 days of harvest. Group 3 fungicide. 12-hr reentry.
- Prolivo 300 SC at 4 to 5 fl oz/A. Can be used day of harvest. Group 50 fungicide. 4-hr reentry.
- Quintec at 4 to 6.6 fl oz/A. A surfactant is not required when used alone but a nonionic surfactant is preferred if needed for tank-mixes. Do not apply within 21 days of harvest. Group 13 fungicide. 12-hr reentry.
- Rally 40 WSP at 3 to 5 oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 24-hr reentry.
- Regalia at 1 to 4 quarts/A plus another fungicide. Use on 7-day intervals. May be used day of harvest. Group P5 fungicide. 4-hr reentry. O
- Rex lime sulfur (28%) at 0.75 to 1 gal/100 gal water. 48-hr reentry. O
- Sovran at 3.2 to 4.8 oz/A. Rotate with other fungicides that have different modes of action. Do not use within 14 days of harvest. Do not use organosilicate surfactants. Some sweet cherries, such as 'Van', may be injured if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
- Spectracide Immunox at 1.25 fl oz/gal water. Do not use within 2 weeks of harvest or more than five (5) times per season. H
- Sulfur products. Do not extend intervals beyond 14 days. Sulfur can burn foliage when applied above 85°F. The temperature relationship is correlated with increases in the daily maximum, within a few days after application. Grapes in California can withstand sulfur applications (at lower rates) above 85°F if there is no major short-term changes in the daily maximum. May injure labrusca types like 'Concord'. Do not use within two weeks of an oil spray. Hydrogen sulfide in the wine may increase if used within 35 days of white-wine harvest or within 50 days of red-wine harvest. Wettable powdered formulations are less likely to result in hydrogen sulfite in the wine than micronized formulations. Addition of a spreader sticker adjuvant may be helpful. Group M2 fungicides. 24-hr reentry. O
- Cosavet-DF (80% sulfur) at 2 to 5 lb/A.
- Kumulus DF (80% sulfur) at 2 to 10 lb/A.
- Microthiol Disperss (80% sulfur) at 3 to 10 lb/A. Do not use a spreader-sticker. Use at 7- to 14-day intervals.
- Safer Garden Fungicide (12% sulfur) at 2 fl oz/gal water. Thoroughly sprayed over the entire plant. Do not use within 21 days of harvest. H
- Tebuconazole-based fungicides are registered. Do not apply within 14 days of harvest. Group 3 fungicides. 12-hr reentry.
- Orius 20 AQ at 8.6 oz/A.
- Tebucon 45 DF at 4 oz/A.
- TopGuard at 8 to 10 fl oz/A. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr to 5-day reentry depending on activity.
- Topsin M WSB at 1 to 1.5 lb/A plus another fungicide. Do not use within 14 days of harvest. Resistant fungi make this product ineffective and may be prevalent in many vineyards. Group 1 fungicide. 2-day reentry.
- Torino at 3.4 oz/A. Do not use within 3 days of harvest. Group U6 fungicide. 4-hr reentry.
- Trionic 4 SC at 4 to 8 fl oz/A. Do not use within 7 days of harvest. Group 3 fungicide. 12-hr reentry.
- Vacciplant at 14 to 60 fl oz/A plus an effective fungicide. Can be used day of harvest. Unknown efficacy in the PNW. Group P4 fungicide. 4-hr reentry.
- Vivando at 10.3 to 15.4 fl oz/A. Do not use with horticultural oils, or use within 14 days of harvest. Group 50 fungicide. 12-hr reentry.
- Aprovia Top at 8.5 to 13.5 fl oz/A. Do not apply within 21 days of harvest. Group 3 + 7 fungicide. 12-hr reentry.
- Inspire Super at 16 to 20 fl oz/A. Do not use within 14 days of harvest or on 'Concord'. Group 3 + 9 fungicide. 12-hr reentry.
- Luna Experience at 6 to 8.6 fl oz/A. Do not use on 'Concord' or within 14 days of harvest. Group 3 + 7 fungicide. 12-hr to 5-day reentry depending on activity.
- Luna Sensation at 4 to 7.6 fl oz/A. Do not use for powdery mildew control if already used for bunch rot control. Do not use on 'Concord' or within 14 days of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Merivon at 4 to 5.5 fl oz/A. Do not mix with any other materials, use within 14 days of harvest, or on cultivars such as Concord or Niagra. Group 7+11 fungicide. 12-hr reentry.
- Miravis Prime at 9.2 to 13.4 fl oz/A. Do not use for powdery mildew control if already used for bunch rot control. Do not use within 14 days of harvest. Group 7 + 12 fungicide. 12-hr reentry.
- Pristine at 8 to 12.5 oz/A. Do not use within 14 days of harvest, or more than five (5) times/year. Do not use on labrusca type grapes such as 'Concord' due to possible foliar injury. Group 7 + 11 fungicide. 12-hr or 5-day reentry based on activity.
- Quadris Top at 12 to 14 fl oz/A. Do not apply within 14 days of harvest or on 'Concord' grapes. Group 3 + 11 fungicide. 12-hr reentry.
- Topguard EQ at 5 to 6 fl oz/A. Do not use with silicone surfactants or within 14 days of harvest. Sprayers should not be used on apples. Group 3 + 11 fungicide. 12-hr reentry.
- Unicorn DF at 1.75 to 2.5 lb/A plus a non-ionic surfactant. Includes sulfur in the formulation. Group M1 + 3 fungicide. 24-hr reentry.
Notes If you are trying to bring an abandoned vineyard back into production, spraying lime sulfur during the dormant season or micronized sulfur at 100% budbreak may help bring powdery mildew under control with a normal, season-long spray program.
Although OxiDate is registered, it will not control this disease due to its short residual.
Although Revus Top is registered, it contains a chemical for control of downy mildew, a disease not found on grape in the PNW and thus is not recommended for use.
Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Intuity.
Forecasting Several forecasting programs are available for scheduling fungicide applications. The standard Oregon phenology-based program begins applications at 6 to 8 inches shoot growth and continues at regular intervals based on grapevine development. The Gubler-Thomas (UC-Davis) program uses leaf wetness and temperature early in the year to predict ascospore infection periods and only temperature during the summer to predict conidial infection periods. The New York (Gadoury) program is based on rainfall and temperature. The Kast (Oi Diag) program incorporates relative humidity along with temperature and rainfall. All programs have been effective at timing fungicides and controlling powdery mildew in western Oregon. Additional models have been developed in Canada (Carisse) and Italy (Caffi). An ascospore release model for western Oregon incorporates leaf wetness, temperature, precipitation, and relative humidity but was only 66% accurate. The use of rose bushes at the end of rows is based only on anecdotal accounts and has no basis in the scientific literature.
Biological control Short intervals such as 7 days between applications is recommended especially during bloom. Generally more effective when disease-pressure is low. More effective when combined with leaf removal practices and/or integrated with synthetic fungicides at bloom.
- Actinovate AG (Streptomyces lydicus strain WYEC 108) at 3 to 12 oz/A plus a spreader-sticker. California trials averaged 50% control in 5 trials. 1-hr reentry. O
- Bayer Advanced Natria Disease Control RTU (Bacillus subtilis strain QST 713) is registered for the home garden. Active ingredient is a small protein. Ineffective as a standalone treatment based on tests in western Oregon. H O
- DoubleNickel 55 (Bacillus amyloliquefaciens strain D747) at 0.25 to 3 lb/A. Poor control in western Oregon. Group 44 fungicide. 4-hr reentry. O
- LifeGard WG (Bacillus mycoides isolate J) at 4.5 oz/100 gal water. Unknown efficacy. 4-hr reentry. O
- Prevont (Bacillus subtilis strain IAB/BS03) at 10 to 30 fl oz/100 gal water. Unknown efficacy. Preharvest interval not specified. 4-hr reentry. O
- Serenade ASO (Bacillus subtilis strain QST 713) 2 to 4 quart/A. Active ingredient is a small protein. Serenade Garden Disease Control is available for home use. Ineffective as a standalone treatment based on tests in western Oregon. 4-hr reentry. H O
- Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A plus a spreader-sticker. May be applied up to and including the day of harvest. 4-hr reentry. O
References Gadoury, D.M., Seem, R.C., Ficke, A., and Wilcox, W.F. 2003. Ontogenic resistance to powdery mildew in grape berries. Phytopathology 93:547-555.
Kwasniewski, M. T., Sacks, G. L., and Wilcox, W. F. 2014. Persistence of elemental sulfur spray residue on grapes during ripening and vinification. American Journal of Enology and Viticulture, 65:453-462.
Thiessen, L.D, Neill, T.M., and Mahaffee, W.F. 2018. Assessment of Erysiphe necator ascospore release models for use in the Mediterranean climate of Western Oregon. Plant Disease 102:1500-1508.